Several patents describe the 2,2,6,6-tetraalkyl-4-oxopiperidines and their synthesis and the following patents are representative:
U.S. Pat. No. 3,959,295 discloses the preparation of triacetone amine by reacting acetone and ammonia in the presence of an acidic catalyst, i.e., Lewis acids, protonic acids, and their salts with ammonia or with organic bases such as boron trifluoride, ammonium chloride or sulfuric acid. The process comprises two steps i.e., the first wherein ammonia is reacted with ammonia in the presence of an acid catalyst in an amount from 0.2 to 12 mole % relative to the acetone at a temperature of from 5.degree. to 60.degree. C. and the second, completing the reaction with or without the addition of acetone but using a total amount of acetone of 1.6 moles or greater per mole of ammonia.
U.S. Pat. No. 4,275,211 discloses a process of producing tetraalkyl-4-oxopiperidines such as triacetone amine by effecting reaction between acetone and ammonia at temperatures from 25.degree.-55.degree. C. in the presence of a strongly acid ion exchanger having a medium or large mesh size or having large macropores. These strongly acidic cations exchangers are based mainly on styrene with divinylbenzene as a crosslinking agent and then sulfonated. Lewatit SC 104/H resin is an example of such a strong acid cation exchanger. It was acknowledged that ion exchange catalysts had been used in the past (U.S. Pat. No. 3,904,625) but such processses were of little success.
U.S. Pat. No. 3,943,139 discloses a process for producing triacetone amine by reacting phorone and ammonia under pressures from 1-3 bars and temperatures from 60.degree.-110.degree. C.
U.S. Pat. No. 3,959,298 discloses a process for preparing triacetone amine by reacting acetonine with water in the presence of an acid catalyst present in an amount to provide at least 12.5 mole % based on the acetonine. Examples of acid catalysts include carboxylic acids, mineral acids, organic sulfur-oxygen acids or organic phosphorus-oxygen acids. Specific examples include saturated and unsaturated monobasic aliphatic acids such as acetic acid; methylsulfuric acid and sulfonic acids such as methane sulfonic acids, benzene sulfonic acids and the phosphorus acids such as diethyl-phosphonic acid or benzene phosphinic acid.
U.S. Pat. No. 3,904,625 discloses a process for preparing 2,3,4,5-tetrahydropyrimidines (THP) by reacting a carbonyl compound such as a ketone or aldehyde with ammonia in the presence of a ion exchange catalyst preferably an anionic resin such as a sulfonated resin commonly sold under the trademark AMBERLITE.RTM.. Temperatures of reaction range from 0.degree. to 100.degree. C. preferably from 10.degree. to 40.degree. C. with reaction times ranging from 1 to 3 hours typically.